I dealt with this question by measuring the diameter of my largest dry bag -- in my case my synthetic sleeping bag -- to make sure that it fit through the hatch opening. I also played around with the placement of the hatch. Since I wanted the bag to be up against the bulkhead (the only place where I could stuff it so that the bag's long axis was perpendicular to the boat's axis), I placed the hatch opening about six or eight inches behind the bulkhead, allowing me to cram the sleeping bag into the forward end of the hatch, and the rest of my gear aft of the bag (and easy to get to without having to take out the sleeping bag in case I just want a tool, the first aid kit, etc.). As long as the hatch is wide enough for your widest bag, you're doing OK. The length of the hatch is probably a little less important. I've got several long bags -- sleeping pad and tent poles -- that are easy to get out even though they are longer than the hatch. Think through your usual set of bags and where you want your weight located, and plan the hatch size (and placement) accordingly.

Remember to put a leash on the hatch. They have a tendency to float away when you've left them sitting next to the boat while you have lunch or set up the tent.

Jim